A. Field of Invention
This invention relates to and discloses thermoplastic vulcanizate compositions having improved adhesion to polar materials and substrates, without requiring pre-treatment, or pre-heating of the polar materials or the use of additional adhesives.
B. Description of the Related Art
Thermoplastic vulcanizates (“TPVs”) are a fine dispersion of highly vulcanized rubber particles in a continuous phase of a thermoplastic, which is typically a polyolefin. TPVs are traditionally made by blending a rubber with a semi-crystalline thermoplastic under conditions that allow for the dynamic vulcanization of the rubber. The result is a material comprised of a continuous plastic phase formed by the polyolefin and interspersed with discrete, crosslinked rubber particles, which form a rubber phase. TPVs have the benefit of the elastomeric properties provided by the rubber phase, with the processability of thermoplastics.
It is often desirable to overmold polar substrates, including metal and plastics such as nylon, with TPVs. However, TPVs are typically non-polar, which presents challenges when overmolding TPVs onto polar substrates, as adhesion between non-polar TPVs and polar substrates is generally poor. Existing methods of improving TPV adherence to polar substrates involve pre-treating the substrate surfaces to improve adhesion or bonding. Pretreatment methods may include solvent etching, sulfuric acid or chromic acid etching, sodium treatment, ozone treatment, flame treatment, UV irradiation, and plasma treatment. These procedures are costly, use hazardous materials, result in product degradation, and create environmental hazards.
Other approaches to improving the adherence of TPVs to polar substrates include the use of adhesives, and “mechanically” locking the TPV layer onto the substrate. The use of adhesives increases cost and production time. Mechanical approaches to improving adherence involve processing or specially forming the substrate, for example, by forming channels in the substrate into which the TPV can flow, solidify, and thereby lock the TPV layer in place the substrate.
Still other efforts include those described in U.S. Pat. No. 4,957,968 in which adhesive thermoplastic elastomer compositions comprised of blends of an elastomeric copolymer or terpolymer, polyolefin, and an amount of a chemically modified polyolefinic polymer, exemplified by maleic grafted anhydride polypropylene, are applied to substrates.
Preparation of TPVs with improved high temperature elastic recovery over conventional TPVs (iPP/EPDM), by the dynamic vulcanization of ethylene/alpha-olefin rubber in maleic anhydride grafted polyolefins is described in U.S. Pat. No. 6,028,142.
WO 95/26380 involves the blending of so-called adhesion promoters to a preformed thermoplastic elastomer. According to this patent publication, the adhesion promoter is the reaction product of a functionalized polyolefin (such as maleated polypropylene) and a polyamide. This grafted nylon-MAH-PP blended with the selected thermoplastic elastomer to yield the so-called adhesive product.
U.S. Pat. No. 6,300,418 discloses functionalized rubber, exemplified by a maleic anhydride graft thermoplastic rubber, rather than functionalized polyolefin (as in the WO 95/26380 publication), which is blended with a polyamide to form an adhesion promoter that is in turn blended, in amounts of between 2 and 60 weight %, with a thermoplastic elastomer composition.
All of the above-mentioned methods at improving adhesion suffer drawbacks that limit the overall combination of adhesive properties and physical properties. One deficiency, for example in WO 95/26380, is that blending the nylon/PP-g-MAH melt blend with the selected thermoplastic elastomer increases the overall hardness of the resulting thermoplastic elastomers. In order to maintain a desirable level of hardness, additional additives to the composition, such as a block copolymer of styrene/conjugated diene/styrene, are used. Compensating for the increase in hardness by starting with or adding amounts of a lower hardness thermoplastic elastomer may result in a product exhibiting an inferior combination of physical and adhesive properties.
Additionally, in the above mentioned efforts, heating of the substrate is often a predicate step to achieving good adherence of the TPV to the substrate. Heating the substrate prior to overmolding is not always practical and adds additional manufacturing steps that increase cost and time. It would be desirable to achieve good adherence without having to heat the substrate.
Finally, many of the above referenced approaches are premised on the addition of materials, sometimes referred to as adhesion promoters, to existing TPVs or blends of unmodified elastomer and unmodified polyolefin. As noted, commercially available TPVs comprising unmodified or non-functionalized elastomer and plastic phases generally have poor adherence to polar substrates. The presence of significant amounts of unmodified elastomer or plastic will limit the adherence that can be achieved merely by dilution with functionalized polyolefin.
It would also be desirable to provide TPV compositions having a plastic phase that imparts a relatively low melt viscosity to the TPV so as to improve the flow of the TPV over the substrate and so as to improve the physical contact of the TPV to the substrate. Additionally, it would be desirable to provide TPVs having a relatively slow crystallization rate so as to maximize contact time between the TPV and the substrate surface prior to crystallization.